Test systems requiring a constant voltage characteristic derived from multiple parallel-coupled loads are sometimes difficult to implement with commercially available loads. For instance, some systems allow only one load to operate in a constant voltage mode, while the remaining loads must be set to a constant current value. This particular implementation, however, limits the dynamic range of total load current because the loads set to the constant current value establish a minimum current for the system.
Other systems set one load in a constant voltage mode (e.g., a master) and drive additional loads (e.g., slaves) in a constant current mode with external control signals. This system, however, can only provide approximate current sharing among the loads. Still other systems provide a more uniform current sharing capability with the use of internal control mechanisms having the ability to convert from constant current control to constant voltage control. It is necessary with such systems to reset the loads during the transition from constant current control to constant voltage control. Resetting the loads from a constant current mode to a constant voltage mode, however, requires a great deal of time and effort that may be better served testing the device under test (DUT).
A typical application that employs commercial test loads for a DUT is with respect to the testing of power supplies and, in particular, a DC power supply (also, referred to as a DUT). A DC power supply, typically, has both a regulated voltage region and an unregulated voltage region where at higher currents the device goes into a current limit mode. The current limit mode is employed to protect the device from the possibly harmful effects of overloads. In the process of testing the DC power supply, it is generally desirable to employ constant current control for the loads when the DUT is in the regulated region and constant voltage control for the loads when the DUT is in the unregulated region.
Applying the aforementioned modes of control for the regulated and unregulated regions of the DUT can be better understood with reference to the voltage/current (V/I) curve of FIG. 3. Attempting to apply a constant voltage condition in the voltage regulated region, for example, makes it difficult (if not impossible in practical applications) to obtain a stable operating point on the curve. This is because the V/I curve of the DUT and the V/I curve of the load are nearly parallel with one another, and noise or drift may prevent a predictable operating condition. Conversely, attempting to establish a constant current condition in the unregulated region of the DUT (where the V/I curve of the DUT may exhibit a near constant current characteristic) also causes difficulties in achieving a stable operating point on the curve. Once again, this is because the V/I curves of the DUT and the load are nearly parallel to one another.
Another acceptable method of establishing stable load conditions in the unregulated region of the DC power supply is to provide constant resistance loads. Multiple constant resistance loads can be connected in parallel to exhibit a net combined parallel resistance.
Although the previously systems provide feasible alternatives for testing a DUT, there are several limitations associated with such systems. For instance, some test specifications may only allow constant voltage mode control as an acceptable method for testing the DUT. In such instances, a combination of current mode and voltage mode control is not available and in view of the foregoing discussion of testing a DC power supply that limitation is not acceptable. Additionally, limitations of some commercially available loads (e.g., some constant resistance loads) preclude the combined loads from achieving an output voltage near zero volts (i.e., a virtual short circuit). Thus, the DUT cannot be tested over its full range as is necessary with many DUTs such as DC power supplies.
Accordingly, what is needed in the art is a system for testing a DUT capable of operating in a regulated region and an unregulated region that allows transitioning between constant current conditions and constant voltage conditions.